Mice homozygous for lpr and gld develop profound lymphadenopathy characterized by the accumulation of two functionally impaired t cell subsets, a predominant B220+ CD4- CD8- (DN) population and a minor CD4+B220+ subset. Previous studies of diseased lpr and gld mice revealed that enlarged LN also contain elevated numbers of conventional T and B cells and an increased proportion of memory-like CD4+ B220- T cells. In the past year we have continued to explore the effects of lpr and gld on T cell selection, growth and activation. These studies revealed that primed CD4+ and CD8+ T cells are detected in increased numbers in lpr LN by 4 wk of age and accumulate progressively thereafter. Two types of memory cells distinguished by differences in ontogeny, phenotype and the spectrum of cytokines secreted post-stimulation were identified. Examination of TcR Vbeta gene utilization revealed no skewing of the Vbeta repertoire that would implicate autoantigens as a primary stimulus for the activation of CD4+ or CD8+ T cells early or late in disease. The pattern of expression of CD69 on B220+ DN T cells provided further evidence that these cells may be chronically stimulated in vivo but unable to progress through the cell cycle. The functional anergy of DN T cells was reversed by cross-linking TcR alpha/beta and CD28 in the presence of PMA, indicating that the blockage in signal transduction via the TcR complex can be overcome in the presence of a strong costimulatory signal. One unifying hypothesis to explain lymphoproliferative disease is that the lpr and gld mutations result in defective regulation of the size of the pool of primed T and B cells. DN T cells may arise from chronically stimulated, abnormally long- lived CD4+ and CD8+ cells that down-regulate the expression of CD4, CD8 and TcR alpha/beta, switch to the B220 isoform of CD45, and become resistant to activation. Selective accumulation of DN T cells may result from a combination of events including chronic low level stimulation, increased lifespan and the ready availability of growth factors. Experiments to test these predictions are in progress and are outlined.